1. Field of the Invention
The present invention relates to squelch systems which operate to turn off the audio output of a radio receiver when there is no received signal and to turn it on when the desired signal is present.
2. Description of the Prior Art
Squelch systems are well known in the art. An example of a present squelch system is shown in FIG. 1 wherein an AM receiver is shown having an input antenna 12 receiving broadcast signals and presenting them to an RF filter 14. Filter 14 removes all but a selected frequency and this signal is amplified by an RF amplifier 16 to produce an amplified selected RF signal. A first mixer 20 in conjunction with a first local oscillator 22 receives the amplified selected RF signal and converts it to a first intermediate frequency signal which is presented to an IF filter 24 to pass only the selected first IF frequency. This signal is amplified by IF amplifier 26 and presented to a second mixer 30 which, in conjunction with a second local oscillator 32, converts the first selected IF frequency to a second IF frequency. The second selected IF frequency is filtered by an IF Filter 34 and amplified by IF amplifier 36 to produce the desired AM signal on output line 37. It should be noted that a xe2x80x9csingle conversionxe2x80x9d receiver architecture could be used in which case the first mixer 20 and oscillator 22 could produce the selected IF frequency to filter 34 and amplifier 36 and the second mixer 30 and local oscillator 32 would not be required. However, the dual conversion (or even a triple conversion) receiver architecture has certain advantages (unrelated to the present invention) and is used in this embodiment. In any event, the signal on output line 37 is presented to envelope detector 40 to produce the audio signal to be amplified by an audio amplifier 42 and filtered by audio filter 44 which removes some of the noise and non-speech audio that may exist in the detected signal. This signal is presented to an audio switch which, as will be explained, operates as a squelch switch to provide an audio output on a line 47 when there is a signal and to prevent the output on line 47 when there is no signal. More particularly, to provide the squelch function, audio switch 46 is enabled and disabled by the output of an OR gate 48. When OR gate 48 produces an output signal, switch 46 is enabled and the audio signal from filter 44 is passed through to a final audio amplifier 49 for producing an audio output to speakers or head phones (not shown) as desired. This output signal from OR gate 48 is produced when the signal level is above the carrier squelch threshold or when the signal has sufficient quieting to be below the noise squelch threshold.
The output of audio amplifier 42 is also presented to a summing circuit 50 which also receives an automatic gain control threshold signal on a line 51. The difference is sent to an integrator 52 to produce the AGC signal for use in controlling the gain of IF amplifier 36, and, through break point amplifiers 54 and 56, controlling the gain of IF amplifier 26 and RF amplifier 16, respectively, in order to get a constant level from the envelope detector 40 that does not depend on the signal level from the antenna 12.
A carrier squelch comparator 60 also receives the AGC signal from integrator 52 on a line 61 and compares it with a predetermined carrier squelch threshold signal on a line 62. Since the AGC voltage for a given receiver gain is an estimation of the signal level, it may be used to determine if the signal at the antenna 12 is above or below the predetermined threshold. If the AGC signal is above the threshold signal, a signal is presented by carrier switch comparator 60 on a line 62 to OR gate 48 which then enables audio switch 46 thus turning the audio output on.
The second IF signal from IF amplifier 36 (or the first IF signal from IF amplifier 26) may be used to provide a noise squelch function as follows: the output from IF amplifier 36 on a line 64 (or the output from IF amplifier 26 on a line shown as dashed line 66) is presented to an FM discriminator 70 which performs an FM demodulation on the IF signal. When there is no signal on line 64 (or line 66), there is a lot of noise from the FM discriminator 70 on a line 71. Similarly, when there is a signal on line 64 (or line 66) then the noise on line 71 decreases. The amount of noise on the output of FM discriminator 70 is thus an indication of the signal strength for the noise squelch circuit. A high pass filter 72 receives this noise signal and filters away the speech frequencies and leaves only the noise at its output 73 which is presented to a noise rectifier 74 which converts it to a signal on line 75 that is proportional to the noise voltage. A noise squelch comparator 76 receives the signal on line 75 and compares it to a predetermined noise squelch threshold signal on a line 77. If the noise voltage on line 75 is less than the threshold value on line 76, a signal on a line 78 is presented to OR gate 48 and the audio switch 46 is enabled and the audio output is turned on. Thus the audio output is turned on either when the signal received by the antenna is strong or the noise level is low. Stated differently, the OR gate 48 enables the audio when the signal power, as measured by the AGC loop, is above a threshold or when the FM noise is below a threshold.
This circuit works well with channel spacing of 25 kilohertz because only 8 kHz is required for transmission of audio information in speech. The remaining bandwidth, above the speech frequencies but below the channel limits, has been used by the noise squelch. Unfortunately, new requirements for airborne very high frequency communications have produced much narrower bandwidth channels which has resulted in splitting each of the 25 kHz channels into three 8.33 kHz channels. While this is sufficient for spoken communications, the band of frequencies used by the noise squelch has been eliminated.
The present invention uses a coded signal, for example, a low frequency (sub-audible) FM tone, to modulate one of the local oscillators and an FM demodulator at the output of the receiver is fed to a narrow bandwidth filter that is tuned to the sub-audible tone frequency. When a desired signal is present, an FM modulated tone will be detected by the FM demodulator. When a desired signal is not present, the attempt to modulate the noise with the sub-audible tone will only produce more noise and consequently, only noise will be detected by the FM demodulator. The presence of the tone, indicative of there being a desired signal, is compared to a threshold value and used to enable the receiver. The absence of a tone, indicative of there being no desired signal, will disable the receiver. The normal communication path of the radio contains a high pass filter that strips off any of the tone were it to manifest itself in the AM detector.